The invention is concerned with a ceramic ferroelectric material having, as one application, a dielectric in capacitors. It is particularly concerned with a ferroelectric system having a stabilized perovskite phase.
A ferroelectric compound, such as PbMg.sub.1/3 Nb.sub.2/3 O.sub.3, has attractive properties for application as a dielectric. These include a high dielectric constant and a broad or diffuse ferroelectric to paraelectric phase transition.
Formation of this compound is difficult because (1) relatively high temperatures, initiating at about 800.degree. C., are required for synthesis, thus hazarding substantial PbO loss, and (2) a metastable and persistent pyrochlore (Pb.sub.3 Nb.sub.4 O.sub.13) phase forms at lower sintering temperatures.
In Paper T4 of Proceedings of the Japan-US Study Seminar on Dielectric and Piezoelectric Ceramics (1982), the author, K. Furukawa, suggested adding excess MgO to the starting powder to convert pyrochlore to perovskite. Subsequently, Swartz and Shrout, in an article in Materials Research Bulletin, (1982), suggested reacting MgO and Nb.sub.2 O.sub.5 and combining the reaction products with PbO in proper proportion.
We have found that these expedients do indeed reduce formation of the pyrochlore phase, but that they do not completely suppress it. Further, if the sintered ferroelectric must be reheated in subsequent operations, there is a tendency for pyrochlore to reform at the expense of perovskite. Thus, it would be desirable to find a means of completely suppressing, and keeping suppressed, formation of the destabilizing pyrochlore.
Most dielectrics based on PbO are designed to be fired at a relatively low temperature in the range of 800.degree.-1000.degree. C. in order to be compatible with Ag or Ag-Pd electrodes. In contrast, however, a fugitive carbon type electrode requires a dielectric material capable of being fired at higher temperatures. This electrode is produced with dispersed carbon which is burnt out to leave a porous structure that is impregnated with a lead alloy. It must be fired at temperatures well above 1000.degree. C. to insure against carbon residue, and consequent poor electrical characteristics.